1. Field of the Invention
The present invention relates to a process for hydrodealkylating alkyl-substituted aromatic hydrocarbons in the presence of a multi-metal catalyst comprising a particular alumina support.
2. Description of the Prior Art
Two types of processes for hydrodealkylating alkyl-substituted aromatic hydrocarbons in a hydrogen atmosphere are presently known, that is, thermal processes and catalytic processes.
According to the first type of process, a thermal hydrodealkylation is effected in the absence of any catalyst. The need for very high reaction temperatures and pressures is an important disadvantage of such processes.
The second type of process is carried out in the presence of catalysts comprising chromium oxide, such as described in the U.S. Pat. No. 2,853,348, and usually requires lower reaction temperatures than those utilized in the thermal processes, yet same reflects the disadvantage of requiring a much too high pressure of hydrogen and the catalyst lacks stability; therefore, such processes of the second type are only of little interest in actual practice.
Among the catalytic processes employing a catalyst containing a Group VIII metal, those utilizing rhodium may be mentioned. Rhodium has long been known as being a metal which favors hydrogenolysis and hydrodealkylation. Its use in hydrodealkylation processes as an active catalyst component, in combination with a certain number of other metals, too has already been disclosed.
Thus, the U.S. Pat. No. 2,734,929 discloses the use of a catalyst comprising an alumina support and, as an active component, the combination of a metal of Group VIB and a metal of Group VIII, preferably molybdenum or cobalt.
The U.S. Pat. No. 3,306,944 discloses a hydrodealkylation catalyst comprising an alumina support and, as an active component, rhodium, ruthenium, osmium or iridium.
The U.S. Pat. Nos. 3,686,340 and 3,825,503 disclose a hydrodealkylation catalyst comprising an alumina support and, as an active component, a composition including three components comprising a noble metal of Group VIII or nickel, tin oxide or lead oxide, an alkali metal oxide or an oxide of an alkaline earth metal or a rare earth metal.
The U.S. Pat. No. 3,204,006 discloses a hydrodealkylation catalyst comprising a single noble metal of Group VIII, preferably platinum on alumina.
The U.S. Pat. No. 3,213,153 discloses a catalyst for hydrodealkylating alkyl naphthalenes which comprises, as an active component, a combination of a metal of Group VIII and a metal of Group VIA.
These prior art hydrodealkylation processes which use various catalytic formulations based on rhodium have major disadvantages. On the one hand, relatively elevated temperatures and pressures are needed for conducting same; on the other hand, they exhibit poor selectivity.
Furthermore, the stability of these catalytic formulations is usually low.